Field Plates on Two Opposed Surfaces of Double-Base Bidirectional Bipolar Transistor: Devices, Methods, and Systems

ABSTRACT

Dual-base two-sided bipolar power transistors which use an insulated field plate to separate the emitter/collector diffusions from the nearest base contact diffusion. This provides a surprising improvement in turn-off performance, and in breakdown voltage.

CROSS-REFERENCE

Priority is claimed from U.S. application 62/063,090, filed 13 Oct.2014, which is hereby incorporated by reference.

In the United States, continuation-in-part priority is also claimed fromcopending PCT application WO2014/210072 (which designates the US), andtherethrough to provisional U.S. application 61/838,578 filed 24 Jun.2013, which is hereby incorporated by reference.

BACKGROUND

The present application relates to Double-Base Bidirectional Bipolartransistors, and more particularly to power transistors of the generaltype known as “B-TRANs.”

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

Published U.S. application US 2014-0375287 disclosed a fullybidirectional bipolar transistor, having emitter/collector regions onboth faces of a semiconductor die, and also having base contact regionson both faces. In one group of embodiments (shown e.g. in FIG. 13A ofthat application, and described in paragraph [0083]), theemitter/collector regions are laterally separated from the base contactregions by a dielectric-filled trench. This reduces same-side carrierrecombination in the ON state.

Application US 2014-0375287 also describes some surprising aspects ofoperation of the device. Notably: 1) when the device is turned on, it ispreferably first operated merely as a diode, and base drive is thenapplied to reduce the on-state voltage drop. 2) Base drive is preferablyapplied to the base nearest whichever emitter/collector region will beacting as the collector (as determined by the external voltage seen atthe device terminals). 3) A two-stage turnoff sequence is preferablyused. 4) In the off state, base-emitter voltage (on each side) islimited by a low-voltage diode which parallels that base-emitterjunction.

A somewhat similar structure was shown and described in applicationWO2014/122472 of Wood. However, that application is primarily directedto different structures. The Wood application also does not describe themethods of operation described in the US 2014-0375287 application. TheWood application also does not appear to describe lateral trenchisolation between emitter/collector regions and base contact regions.

The present application provides improvements in structures of thistype, in methods of operating such structures, and in systems whichincorporate such structures.

The present application teaches, among other innovations, asymmetrically bidirectional dual-contact base bipolar junctiontransistor, in which both of the emitter/collector regions, which arepresent on both (opposed) surfaces of a semiconductor die, aresurrounded by a field plate in a trench. This field plate separates theemitter/collector regions from adjacent base contact regions. Since basecontact regions are present on both surfaces of the device, thisstructure improves the breakdown voltage of whichever base contactregion is on the collector side, without degrading the characteristicsof the base contact region on the other side. This provides surprisingimprovement in the breakdown voltage of a device like that of US2014-0375287.

The present application also teaches, among other innovations, methodsof operating structures of this type, and systems which incorporatestructures of this type.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments and whichare incorporated in the specification hereof by reference, wherein:

FIG. 1 schematically shows an exemplary B-TRAN having trenched fieldplates.

FIG. 2 shows the preferred circuit symbol for a B-TRAN-type device.

FIG. 3 schematically shows one sample embodiment of a B-TRAN havingdielectric-filled trenches, as described in published application US2014-0375287, which is hereby incorporated by reference.

FIG. 4A shows an exemplary doping profile in a device like that ofFIG. 1. FIG. 4B shows the distribution of potential, in a device likethat of FIG. 1, at breakdown.

FIG. 5A shows the electric field, in a device like that of FIG. 1, atbreakdown. FIG. 5B shows impact ionization, in a device like that ofFIG. 1, at breakdown.

FIG. 6 shows the distribution of current density magnitude, in a devicelike that of FIG. 1, at breakdown.

FIG. 7 shows a current density distribution plot, with current vectorsindicated, for the upper side of a device like that of FIG. 1, atbreakdown.

FIG. 8 shows a current density distribution plot, with current vectorsindicated, like that of FIG. 6, at breakdown, for the lower side of adevice like that of FIG. 1.

FIG. 9 shows a plan view of one surface of a device like that of FIG. 1,showing how the emitter/collector areas are laterally surrounded byvertical field plates.

FIG. 10 is a partial cross-section of the periphery of a device likethat of FIG. 1, showing doping profiles generally.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

The class of fully bidirectional bipolar double-base transistorsdescribed in Published U.S. application US 2014-0375287 is now commonlyreferred to as a “B-TRAN.” In some embodiments of that application, theemitter/collector regions are laterally separated from the base contactregions by a dielectric-filled trench. A B-TRAN is a three-layerfour-terminal bidirectional bipolar transistor, as shown in the sampleembodiment of FIG. 1. It is a symmetrical device, meaning that thepolarity of the voltage on the two “end” terminals 102 and 104determines which of these two terminals functions as the emitter (i.e.,has a forward biased junction), and which of these terminals functionsas the collector (i.e., has a reverse biased junction). It typically hasa gain in the range of 5-20, and when biased “off,” is capable ofwithstanding a high voltage in either direction. In addition, as abipolar transistor, it has a minimum on-voltage (V_(CE-SAT)) that can beas low as 0.2 volts, which is well below the voltage drop of a junctiondiode (0.6 volts in silicon). Since the B-TRAN is symmetrical, it can beused in any circuit requiring a bi-directional switch. A device symbolthat shows the symmetry of a B-TRAN is shown in FIG. 2.

A B-TRAN is in the “active off-state” when the e-base (base on emitterside) is shorted to the emitter, and the c-base (base on the collectorside) is open. In this state with an NPN B-TRAN, the collector is theanode (high voltage side), and the emitter is the cathode (low voltageside).

The B-TRAN is also off when both bases are open, but due to therelatively high gain of the B-TRAN in this state, the breakdown voltageis low. The series combination of a normally-ON switch and a diodeattached between each base and its respective emitter/collector, asdisclosed elsewhere, will significantly increase the blocking voltage inthis “passive off-state”. The switches are turned off during normaloperation.

FIG. 3 shows a fully symmetric bidirectional bipolar transistor, inwhich emitter/collector and base contact regions are separated bydielectric-filled trenches. This structure is described in publishedapplication US 2014-0375287. The example shown is an npn device, inwhich n-type emitter/collector regions are present on both front andback faces of a p-type semiconductor die. In addition, separate p-typebase contact regions are provided on both front and back surfaces, toprovide connection to the bulk of the p-type semiconductor material. Inthis structure the whole thickness of the semiconductor die can beregarded as the base, but the two base contact regions are electricallydistinct, and are operated separately.

As the B-TRAN devices are refined and improved, the present inventorshave realized that a double-base bidirectional bipolar transistorpresents several unique challenges. One of these is collector-baseisolation. In normal bipolar junction transistors, the voltagedifference between emitter and base is never very large; however, in afully bidirectional bipolar transistor there are two emitter/collectorterminals rather than a distinct emitter and collector. Whichever sideis acting as the collector (due to the polarity of the externallyapplied voltage) will see a high electric field near the bottom of theemitter/collector diffusion on that side.

The requirement that the same physical structure be capable offunctioning as either an emitter or a collector places significantlydifferent requirements on the same junction at different times. Forinstance, when collector/emitter terminal 102 in FIG. 3 is acting as thecollector, the reverse biased P-N junction at the top of the device mustwithstand the full voltage that is across the device. Simulations haveshown, surprisingly, that the structure of FIG. 3, which usesinsulator-filled trenches 106 to separate the n-type regions from thep-type regions, results in increased field intensity below the bottom ofthe insulation-filled trench, which decreases the withstand or breakdownvoltage of the device.

This reduction in device performance may be reduced or eliminated by thecombination of dielectric layer 108 adjacent to the sidewall and bottomof trench 106, and field plate 110 in the center of the trench connectedto the n-type junction, as shown in FIG. 1.

The thickness of dielectric 108 must be chosen so the intensity of theelectric field below the bottom of trench 106 is reduced to anacceptable level for the device being manufactured. However, simulationshave shown that for a layer of silicon dioxide, a thickness in the rangeof 0.2 μm is sufficient for a device having a 1200 Volt breakdown.

The sample embodiment of FIG. 4A shows a doping profile for a devicelike that of FIG. 1 with 5-μm-deep trenches. This sample embodiment hasa breakdown voltage of 1223 V, and an open c-base voltage of 1195.23 V.FIG. 4B shows an exemplary plot of potential at breakdown for a devicelike that simulated in FIG. 4A.

In the sample embodiment of FIG. 5A, electric field at breakdown issimulated for a device like that of FIG. 1. Similarly, FIG. 5B simulatesimpact ionization at breakdown for an exemplary device like thatsimulated in FIG. 4A.

The sample embodiment of FIG. 6 plots the distribution of currentdensity, near the emitter/collector region acting as a collector, atbreakdown for a device like that of FIG. 1. The current density near theemitter is on the order of I_(E)=100 A/cm². The base-collector voltageis V_(BC)=0.7 V, and the collector-emitter voltage is V_(CE)=0.184 V. Inthe sample embodiment of FIG. 7, this current density distribution issupplemented by vectors showing the direction of current near thecollector of a device like that of FIG. 1. Similarly, the sampleembodiment of FIG. 8 shows current density distribution and currentdirection near the emitter of a device like that of FIG. 1.

FIG. 9 shows a plan view of one surface of a device like that of FIG. 1,showing how the emitter/collector areas are laterally surrounded byvertical field plates.

FIG. 10 is a partial cross-section of the periphery of a device likethat of FIG. 1, showing doping profiles generally. Note that only twofield-limiting rings are shown in this example, but in practice thenumber of field-limiting rings can be much larger, e.g. ten.

The active region contains the emitter regions, each of which issurrounded by a vertical field plate. There are base contact regionsadjacent to the long edge of each emitter region.

From the edge of the active region to the edge of the die, the regionsthat are present are:

-   -   A field plate that surrounds the entire active region. It        extends outward over the P− region at the edge of the active        region, and into the region that has a thick field oxide region.    -   A series of about 10 field limiting rings (FLRs) consisting of a        lightly doped n-type region diffused into the lightly doped        p-type substrate. The FLRs have field plates to spread the        depletion region.    -   An equipotential ring at the outer perimeter of the termination        consisting of an aluminum ring that contacts a P+ region formed        at the die edge.

In this example the dopings shown as n− (in the periphery) arepreferably the same Phosphorus dopings as those used in theemitter/collector regions. The emitter/collector regions themselves alsohave a shallower n+ contact doping, which is not present in then-periphery regions. This is preferably As (arsenic).

Similarly, in this example the dopings shown as p− (in the periphery)are preferably the same boron dopings as those used in theemitter/collector regions. The regions themselves also have a shallowerp+ contact doping, which is not present in the n− periphery regions.

The p−-doping is the bulk doping of the semiconductor die. This can be,for example, 180-250 ohm-cm for a 1200V part, 80-120 ohm-cm for a 600Vpart, and higher for higher rated voltages.

Advantages

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   -   Increased breakdown voltage;    -   Increased ruggedness;    -   Better performance during turn-off; and    -   Improved device performance.

According to some but not necessarily all embodiments, there isprovided: Dual-base two-sided bipolar power transistors which use aninsulated field plate to separate the emitter/collector diffusions fromthe nearest base contact diffusion. This provides a surprisingimprovement in turn-off performance, and in breakdown voltage.

According to some but not necessarily all embodiments, there isprovided: A power semiconductor device, comprising: first and secondfirst-conductivity-type emitter/collector regions, located respectivelyon first and second surfaces of a second-conductivity-type semiconductordie having first and second surfaces; first and secondsecond-conductivity-type base contact regions, located respectively onthe first and second surface of the semiconductor die; first and secondinsulated field plate structures, located respectively on the first andsecond surface of the semiconductor die; wherein the field platestructures are conductive, and are vertically extended, and laterallyadjoin the emitter/collector regions; wherein the firstemitter/collector region is generally shaped like a stripe with sidesand ends, and is laterally surrounded by the first insulated field platestructure at both sides and both ends, and is electrically connected tothe first insulated field plate structure; and wherein the secondemitter/collector region is generally shaped like a stripe with sidesand ends, and is laterally surrounded by the second insulated fieldplate structure at both sides and both ends, and is electricallyconnected to the second insulated field plate structure; whereby thebreakdown voltage is improved under either polarity of applied voltage.

According to some but not necessarily all embodiments, there isprovided: A power semiconductor device, comprising: a p-typesemiconductor die having first and second surfaces; first and secondn-type emitter/collector regions, located respectively on the first andsecond surface of the semiconductor die; first and second p-type basecontact regions, located respectively on the first and second surface ofthe semiconductor die; first and second trenched field plate structures,located respectively on the first and second surface of thesemiconductor die; wherein the first emitter/collector region iselectrically connected to, and is entirely surrounded by, the firsttrenched field plate structure; and wherein the second emitter/collectorregion is electrically connected to, and is entirely surrounded by, thesecond trenched field plate structure; whereby the breakdown voltage isimproved under either polarity of applied voltage.

According to some but not necessarily all embodiments, there isprovided: A power semiconductor device, comprising: an n-typesemiconductor die having first and second surfaces; first and secondp-type emitter/collector regions, located respectively on the first andsecond surface of the semiconductor die; first and second n-type basecontact regions, located respectively on the first and second surface ofthe semiconductor die; first and second trenched field plate structures,located respectively on the first and second surface of thesemiconductor die; wherein the first emitter/collector region iselectrically connected to, and is entirely surrounded by, the firsttrenched field plate structure; and wherein the second emitter/collectorregion is electrically connected to, and is entirely surrounded by, thesecond trenched field plate structure; whereby the breakdown voltage isimproved under either polarity of applied voltage.

According to some but not necessarily all embodiments, there isprovided: A method for switching, comprising: in the ON state, drivingbase current through one, but not both, of first and second p-type basecontact regions which are located on opposite faces of a p-typesubstrate, to thereby allow passage of current between first and secondn-type emitter/collector regions which are also located on oppositefaces of the p-type substrate; wherein the base contact regions and theemitter/collector regions are laterally separated, on both faces of thesubstrate, by a vertically extended conductive field plate which islaterally surrounded by dielectric material; and in the OFF state,reducing peak electric field near the emitter-base junctions bycapacitive coupling to the insulated field plate.

According to some but not necessarily all embodiments, there isprovided: A method for switching, comprising: in the ON state, drivingbase current through one, but not both, of first and second n-type basecontact regions which are located on opposite faces of a n-typesubstrate, to thereby allow passage of current between first and secondp-type emitter/collector regions which are also located on oppositefaces of the n-type substrate; wherein the base contact regions and theemitter/collector regions are laterally separated, on both faces of thesubstrate, by a vertically extended conductive field plate which islaterally surrounded by dielectric material; and in the OFF state,reducing peak electric field near the emitter-base junctions bycapacitive coupling to the insulated field plate.

According to some but not necessarily all embodiments, there isprovided: A method for switching a power bipolar semiconductor devicewhich includes both an n-type emitter/collector region, and also ap-type base contact region, on both first and second surfaces of ap-type semiconductor die, comprising: during the ON state, driving basecurrent into one of the base contact regions; and during transition tothe OFF state, temporarily shorting the base contact region on the firstsurface to the emitter/collector region on the first surface, while alsoshorting the base contact region on the second surface to theemitter/collector region on the second surface, and thereafter floatingat least the base contact region on the first surface; wherein the basecontact region on the first surface is not connected to the base contactregion on the second surface; wherein a trenched field plate separateseach emitter/collector region from the respective base contact region;whereby currents of both polarities are controllably switched betweenthe emitter/collector regions on opposite surfaces.

According to some but not necessarily all embodiments, there isprovided: A switching circuit comprising: a two-base bidirectional npnsemiconductor device which includes both n-type emitter/collectorregions, and also p-type base contact regions, on both opposed surfacesof a p-type monolithic semiconductor die; control circuitry which isconnected separately to the first and second base contact regions on theopposed surfaces; first and second distinct clamp circuits, eachcomprising a series combination of a low-voltage diode and a normally-onswitch, connected so that, when the normally-on switch is on, the anodeof the low-voltage diode is connected to the p-type base contact region,and the cathode of the low-voltage diode is connected to the n-typeemitter/collector region; and insulated field plate structures intrenches separating each emitter/collector region from adjacent basecontact regions; wherein the low-voltage diode turns on at a forwardvoltage which is less than the diode drop of the p-n junction between anemitter/collector region and the semiconductor die; whereby, when thenormally-on switch is ON, the p-n junction between an emitter/collectorregion and the semiconductor die cannot ever be forward biased.

According to some but not necessarily all embodiments, there isprovided: A method for switching a power semiconductor device whichincludes both an n-type emitter/collector region, and also a p-type basecontact region, on both first and second surfaces of a p-typesemiconductor die, comprising: in the ON state, flowing base currentthrough the base contact region which is nearer the more positive one ofthe emitter/collector regions, without flowing base current through theother of the base contact regions; wherein the base contact region onthe first surface is not electrically connected to the base contactregion on the second surface, except through the semiconductor dieitself; wherein the emitter/collector region on the first surface is notelectrically connected to the emitter/collector region on the secondsurface; and wherein a trenched field plate separates eachemitter/collector region from the respective base contact region;whereby bidirectional switching is achieved with low on-state voltagedrop and reliable turn-off.

According to some but not necessarily all embodiments, there isprovided: A method for switching a power semiconductor device whichincludes both an n-type emitter/collector region, and also a p-type basecontact region, on both first and second surfaces of a p-typesemiconductor die, comprising: at turn-on, shorting the more positiveone of the emitter/collector regions together with the base contactregion on the same one of the surfaces, to thereby conduct current witha diode voltage drop; and thereafter flowing base current through atleast one of the base contact regions, to initiate conduction as abipolar transistor with less than a diode voltage drop; wherein the basecontact region on the first surface is not electrically connected to thebase contact region on the second surface, except through thesemiconductor die itself; wherein a trenched field plate separates eachemitter/collector region from the corresponding base contact region;whereby bidirectional switching is achieved with low on-state voltagedrop and reliable turn-off.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

In the above example the semiconductor die is silicon, but othersemiconductor materials can be used instead.

The field plate is preferably made of doped polysilicon, but otherconductive materials, such as aluminum or tungsten, can also be used.

The insulation around the field plate is preferably thick enough towithstand the large voltage drop seen at the collector side in the offstate. Moreover, the insulation around the field plate can be morerobust, if desired.

Preferably the insulation around the field plate is provided by a thinlayer of silicon dioxide which is grown on the sidewalls of a trenchetched into the (silicon) die. However, this layer of insulation canalternatively include multiple different materials.

Additional general background, which helps to show variations andimplementations, can be found in the following publications, all ofwhich are hereby incorporated by reference: U.S. Pat. Nos. 9,029,909,9,035,350, 9,054,706, 9,054,707, 9,059,710.

Additional general background, which helps to show variations andimplementations, as well as some features which can be implementedsynergistically with the inventions claimed below, may be found in thefollowing US patent applications. All of these applications have atleast some common ownership, copendency, and inventorship with thepresent application, and all of them, as well as any material directlyor indirectly incorporated within them, are hereby incorporated byreference: US 2015-0214055 A1, US 2015-0214299 A1, US 2015-0270771 A1,US 2015-0270837 A1, US 2015-0280613 A1; PCT/US14/43962, PCT/US14/69611,PCT/US15/11827; 14/755,065, 14/791,977, 14/792,262; and all priorityapplications of any of the above thereof, each and every one of which ishereby incorporated by reference.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

1-14. (canceled)
 15. A method for switching, comprising: in the ONstate, driving base current through one, but not both, of first andsecond p-type base contact regions which are located on opposite facesof a p-type substrate, to thereby allow passage of current between firstand second n-type emitter/collector regions which are also located onopposite faces of the p-type substrate; wherein the base contact regionsand the emitter/collector regions are laterally separated, on both facesof the substrate, by a vertically extended conductive field plate whichis laterally surrounded by dielectric material; and in the OFF state,reducing peak electric field near the emitter-base junctions bycapacitive coupling to the insulated field plate.
 16. The method ofclaim 15, further comprising, at turn-on, shorting the more positive oneof the emitter/collector regions together with the base contact regionon the same one of the surfaces, to thereby conduct current with a diodevoltage drop.
 17. The method of claim 15, wherein said step of drivingbase current drives base current through the base contact region whichis nearer the more positive one of the emitter/collector regions,without flowing base current through the other of the base contactregions.
 18. The method of claim 15, further comprising, duringtransition to the OFF state, temporarily shorting the base contactregion on one said surface to the emitter/collector region on the samesurface, while also shorting the base contact region on the other saidsurface to the emitter/collector region on the same surface, andthereafter floating at least the base contact region on one of saidsurfaces.
 19. A method for switching, comprising: in the ON state,driving base current through one, but not both, of first and secondn-type base contact regions which are located on opposite faces of an-type substrate, to thereby allow passage of current between first andsecond p-type emitter/collector regions which are also located onopposite faces of the n-type substrate; wherein the base contact regionsand the emitter/collector regions are laterally separated, on both facesof the substrate, by a vertically extended conductive field plate whichis laterally surrounded by dielectric material; and in the OFF state,reducing peak electric field near the emitter-base junctions bycapacitive coupling to the insulated field plate.
 20. The method ofclaim 19, further comprising, at turn-on, shorting the more negative oneof the emitter/collector regions together with the base contact regionon the same one of the surfaces, to thereby conduct current with a diodevoltage drop.
 21. The method of claim 19, wherein said step of drivingbase current drives base current through the base contact region whichis nearer the more negative one of the emitter/collector regions,without flowing base current through the other of the base contactregions.
 22. The method of claim 19, further comprising, duringtransition to the OFF state, temporarily shorting the base contactregion on one said surface to the emitter/collector region on the samesurface, while also shorting the base contact region on the other saidsurface to the emitter/collector region on the same surface, andthereafter floating at least the base contact region on one of saidsurfaces.